Small electrical devices powered by rechargeable batteries are customarily charged at an external charging station. Known in the art are contactless charging stations which have a power supply and which inductively transfer energy from the charging station to the device. To this end, an alternating magnetic field is generated in the charging station by an oscillator which has a coil element and a capacitor element, wherein the coil element simultaneously forms the primary coil of an inductive transmitter and the secondary coil of the transmitter is arranged in the device being charged. For this reason, the charging station is normally designated as the primary side and the device being charged is designated as the secondary side. The oscillator is damped by the energy output via the magnetic field of the transmitter. As a result, the power supply must resupply electrical energy accordingly.
Modern charging stations normally have three operating states. The first state is the operating mode, in which the secondary side demands a constant supply of energy, for example, for operating the device or for charging a rechargeable battery installed in the device.
The second state is the simple standby mode, in which the device is not in the charging station, therefore there is no energy demand at all. The third state is what is known as the extended standby mode, in which the device is located in the charging station but requires energy only occasionally, for example, because the rechargeable battery is fully charged but must occasionally be recharged in order to compensate for self-discharge or the device's own consumption. In this last mentioned case the charging station should switch back and forth between the simple standby mode and the operating mode as needed. The respective operating state of the charging station (primary side) is thus determined by the energy requirements of the small electrical device (secondary side).
It is known to detect the energy requirements of the secondary side directly at the secondary side, transmit corresponding information to the primary and to accordingly adjust the oscillator (meaning, for example, to correspondingly adjust the base emitter voltage of a transistor operating in the oscillator). This solution is rather complex because it requires means for transmitting information from the secondary side to the primary side. As an alternative, the energy requirements of the secondary side might be determined by measuring the energy consumption of the oscillator (primary side) and regulating the oscillator accordingly. However, this variation is ill-suited for adjusting multiple operating states because the energy consumption of the charging station is influenced only minimally by the energy consumption of the device, due to the normally weak coupling between the primary and secondary side of the transmitter.
As such, there is a need to minimize the power consumption of the circuit arrangement in the standby mode for reasons of energy efficiency (see EU Directive 205/32), and therefore to specify a method for the inductive transfer of energy and a circuit arrangement which may be switched in a simple manner to an operating state with reduced power consumption depending on the energy requirements of the secondary side.